If an aeroplane was not stabilized in this way, it would
not only be continually trying to leave its course, but it would
also possess a dangerous tendency to ``nose away'' from the
direction of the side gusts. In such case the gust shown in
the above illustration would turn the aeroplane round the
opposite way a very considerable distance; and the right
wing, being on the outside of the turn, would travel with
greater velocity than the left wing. Increased velocity
means increased lift; and so, the right wing lifting, the
aeroplane would turn over sideways very quickly.
LONGITUDINAL STABILITY.--Flat surfaces are longitudinally
stable owing to the fact that with decreasing angles of
incidence the centre line of pressure (C.P.) moves forward.
The C.P. is a line taken across the surface, transverse
to the direction of motion, and about which all the air forces
may be said to balance, or through which they may be said
to act.
Imagine A to be a flat surface, attitude vertical, travelling
through the air in the direction of motion M. Its C.P. is
then obviously along the exact centre line of the surface
as illustrated.
In B, C, and D the surfaces are shown with angles of
incidence decreasing to nothing, and you will note that the
C.P. moves forward with the decreasing angle.
Now, should some gust or eddy tend to make the surface
decrease the angle, i.e., dive, then the C.P. moves forward
and pushes the front of the surface up.
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